High-resolution photoemission study of Perovskite Mn oxides: Opening of the charge-ordering gap

Electronic structures of the charge-ordering manganite Nd_1-xSr_xMnO₃ (x~0.5) have been studied by resonant and high-resolution photoemission. The Mn 2p-3d (Nd 3d-4f) resonant photoemission confirmed the Mn 3d (Nd 4f) states hybridized with the O 2p states. The high-resolution Mn 3p-3d resonance spectra show a clear transition from the metal to the charge-ordered insulator for x=0.5 upon cooling while a Fermi cut-off is seen for x=0.47 at all temperatures, suggesting that the transition is induced by a subtle competition between the charge-ordering instability and the double-exchange interaction.

Introduction

The manganese-based perovskite oxides have attracted great interest due to the recent discovery of colossal magnetoresistance. These magnetic and transport properties have been explained by the double-exchange (DE) mechanism. Recently, it has been pointed out that the dynamic JT effect is important in addition to the DE interaction. On the other hand, it has been found that some perovskite manganites with commensurate values of x, such as Nd_0.5Sr_0.5MnO₃, undergo a transition from a ferromagnetic metal (FM) to a charge-ordered insulator (COI) upon cooling. The FM-COI transition is accompanied by a spin- and orbital-ordering, and it is also known that a CE-type antiferromagnetic structure is realized in the COI phase. In this paper, we report on resonance and high-resolution photoemission studies on Nd_1-xSr_xMnO₃ with x=0.47 and 0.5 in order to clarify the electronic structures in the whole valence-band region and near the Fermi level (E_F). Nd_0.5Sr_0.5MnO₃ (x = 0.5) undergoes a PI-FM transition at Tc~255 K and a FM-COI transition at T_COI~158 K upon cooling. Nd_0.53Sr_0.47MnO₃ (x = 0.47) is in the FM phase below Tc~275 K and does not exhibit any COI transition.

Results and Discussion

Mn 2p-3d and Nd 3d-4f resonance photoemission of single crystal samples was performed at 300 K at beam line BL-2B of the Photon Factory (PF). The energy resolution was <0.8 and ~1.0 eV, respectively. High-resolution photoemission measurements (hν = 55 eV) were done at beam line BL-3B of PF at various temperatures with the total energy resolution of ~45 meV.
Photoemission spectra of Nd_0.5Sr_0.5MnO₃ in the entire valence-band region are shown in Fig. 1. The spectra taken at hν = 978 (642) and 964 eV (636 eV) correspond to the Nd 3d-4f (Mn 2p-3d) resonance-maximum and -minimum, respectively. A two-peak structure is observed at ~3.5 and ~7 eV in the Nd 3d-4f resonance-maximum spectrum, which is characteristic of a localized 4f³ (Nd³⁺) ground state. From a cluster model analysis, the unhybridized f-hole and ligand-hole energies, and the effective transfer integral have been estimated as 4.7, 5.1, and 1.35 eV, respectively. These results indicate that the O 2p states are located near the bare Nd 4f levels and the Nd 4f-O 2p hybridization effect in the final states is strong. In this way, the contribution of the rare-earth 4f states should be really taken into account for interpreting the valence band structures. In the Mn 2p-3d resonant spectra (hν = 640 and 642 eV), Mn 3d-derived spectral features appear at 0.6, 2.3, and 6.7 eV. The 0.6 eV shoulder and 2.3 eV peak, which are also clarified in the spectrum taken at hν = 55 eV, are ascribed to the e_g and t_2g states, respectively. The 6.7 eV peak corresponds to the t_2g states strongly hybridized with the O 2p states.

Figure 2 shows the temperature-dependent spectra of the x=0.47 and 0.5 samples taken at 55 eV. There are structures at 0.6, 2.3, 3.0, and 5.6 eV in all the spectra. Comparing the spectra with those in Fig. 1, we conclude that the structures at 3.0 and 5.6 eV mainly originate from the O 2p states, which are not prominent in the spectra at the higher photon-energy excitations. For x=0.47, there is no remarkable difference in the spectra between 140 and 170 K, both in the FM phase. As for x=0.5, the gross feature of the spectrum at 140 K is similar to that at 170 K. However, the 2.3 eV structure at 140 K (indicated by an arrow in Fig. 2) is narrower particularly on the lower binding energy side (2.0-1.6 eV) as revealed in the inset. Furthermore, the narrowing of the 2.3 eV structure itself is more distinct for x=0.5 than for x=0.47. We consider that the narrowing is caused by the localization of the t_2g states due to the FM-COI transition. It is known that the t_2g levels are split into two levels by the JT distortion, which is thought to become larger in the COI phase as suggested from the temperature dependence of the lattice parameters. If the energy-splitting in the t_2g levels is remarkably larger in the COI phase than in the FM phase, the 2.3 eV structure would broaden at 140 K. In fact, such a broadening is not observed. Nd_0.5Sr_0.5MnO₃ has the orthorhombic O' structure even in the FM phase, suggesting that t_2g levels are split in contrast to the case of the FM phase of rhombohedral La_0.5Sr_0.5MnO₃. Therefore it is reasonable to conclude that the t_2g levels are already split in the FM phase and that the energy-splitting does not change much between the two phases.

Temperature dependence of the spectra near E_F is shown in Fig. 3. In all the spectra, spectral intensities decrease toward E_F. In the case of x = 0.47, the intensity at E_F is finite at all temperatures and furthermore a clear Fermi cut-off is observed at 20 K. It should be noted that the spectra for x=0.47 have no essential temperature dependence, which is consistent with that Nd_0.53Sr_0.47MnO₃ is in the same FM phase irrespective of the temperature. Contrary to this, the spectra of x = 0.5 have shown distinct temperature dependence between 140 and 170 K. The spectrum at 140 K is suppressed from E_F to 0.4 eV and rather enhanced from 0.7 eV to 1.2 eV compared to that at 170 K. At E_F, the intensity vanishes at 140 K whereas that at 170 K is weak but finite, clearly indicating the opening of the charge-ordering (CO) gap. From the metallic behavior for x = 0.47 and the clear FM-COI transition for x = 0.5, we consider that the transition is induced by a subtle competition between the CO instability, which originates from the commensurate value of x = 1/2, and the DE interaction. Namely, the DE interaction overcomes the CO instability for incommensurate x=0.47, making the e_g band-width large enough to become metallic. From the spectra in the COI phase, the CO gap (the binding energy of the threshold of finite photoemission intensity) is estimated as about 100 meV at 20 K.
Finally we refer to the temperature dependence of the spectra within the FM phase for the perovskite manganese oxide. Our spectra for x=0.47 have little temperature dependence while spectra of La_1-xA_xMnO₃ (A = Sr or Ca) show remarkable temperature dependence. This experimental discrepancy seems to be related to the difference in the crystal structure between the O' phase (Nd_1-xSr_xMnO₃) and the rhombohedral phase (La_1-xSr_xMnO₃). For the rhombohedral manganites, a neutron diffraction study suggests a change of polaron networks with temperature. In the case of Nd_1-xSr_xMnO₃, however, we consider that the "intermediate polaron" can not be formed because of the larger GdFeO₃-type distortion, and consequently there is no temperature change of the polaron networks. It is understandable that the "temperature-independent" spectra of Nd_1-xSr_xMnO₃ within the FM phase are due to the absence of the change of the polaron networks in constast to the La_1-xA_xMnO₃.

Akira Sekiyama